Pompe disease, also known as acid maltase deficiency or Glycogen storage disease type II, is an autosomal recessive metabolic disorder which damages muscle and nerve cells throughout the body. It is caused by an accumulation of glycogen in the lysosome due to a deficiency of the lysosomal acid α-glucosidase enzyme. The build-up of glycogen causes progressive muscle weakness (myopathy) throughout the body and affects various body tissues, particularly in the heart, skeletal muscles, liver and nervous system.
In Pompe disease, a protein, acid α-glucosidase (EC 3.2.1.20), also known as acid maltase, which is a lysosomal hydrolase, is defective. The protein is an enzyme that normally degrades the α-1,4 and α-1,6 linkages in glycogen, maltose and isomaltose and is required for the degradation of 1-3% of cellular glycogen. The deficiency of this enzyme results in the accumulation of structurally normal glycogen in lysosomes and cytoplasm in affected individuals. Excessive glycogen storage within lysosomes may interrupt normal functioning of other organelles and lead to cellular injury. The defective protein is the result of alternative splicing which is caused by mutations in the GAA gene on long arm of chromosome 17 at 17q25.2-q25.3 (base pair chr17:80,101,526 to 80,119,882 build GRCh38/hg38). The gene spans approximately 18 kb and contains 20 exons with the first exon being noncoding.
Although over 460 GAA mutations have been described (http://cluster15.erasmusmc.nl/klgn/pompe/mutations.html), only a few splicing mutations have been characterized. Severe mutations that completely abrogate GAA enzyme activity cause a classic infantile disease course with hypertrophic cardiomyopathy, general skeletal muscle weakness, and respiratory failure and result in death within 1.5 years of life. Milder mutations leave partial GAA enzyme activity which results in a milder phenotype with onset varying from childhood to adult. In general, a higher residual enzyme activity in primary fibroblasts is associated with later onset of Pompe disease. Enzyme replacement therapy (ERT) has been developed for Pompe disease, in which recombinant human GAA protein is administered intravenously every two weeks. This treatment can rescue the lives of classic infantile patients and delay disease progression of later onset patients, but the effects are heterogeneous.
Antisense oligonucleotides (antisense oligomeric compounds, AONs) are currently being tested in clinical trials for their ability to modulate splicing. A classical example is (treatment of) Duchenne muscular dystrophy. In this disease, mutation hotspots are present in certain exons. Using antisense oligomeric compounds, the mutated exon is skipped and the mutation is bypassed. This results in a slightly shorter protein that is still partially functional. It is straightforward to induce exon skipping using antisense oligomeric compounds, because it is evident that the antisense oligomeric compound must be targeted to the relevant splice site. Also in Epidermolysis bullosa (WO2013053819) and in Leber congenital amaurosis symptoms (WO2012168435) antisense oligonucleotides are used for exon skipping.
However, for a very common mutation in Pompe Disease, the so-called c.-32-13T>G (IVS1) mutation, such a strategy does not work. The IVS1 mutation causes a skipping of exon 2 resulting in the deletion of the canonical translation start side and leads to mRNA decay and thus no protein is transcribed. For antisense therapy to work for the IVS1 mutation in Pompe disease, it needs to induce GAA exon 2 inclusion, i.e. an effect strongly contrasting with exon skipping. However, it is very difficult to induce exon inclusion, because it relies on targeting a splicing repressor sequence, which cannot be reliably predicted. Splicing repressor sequences may be present anywhere in the gene, either in an exon (termed exonic splicing silencer or ESS) or in an intron (termed intronic splicing silencer or ISS) and maybe close to the mutation or far away or maybe close to the affected splice site or far away from it.
Our earlier research (e.g. WO 2015/190922 and WO 2015/109021) has led to the discovery of sites in the genomic sequence of the GAA gene that cause aberrant splicing and in these co-pending patent applications it has been shown that antisense oligonucleotide-based compounds directed to those sites may be able to restore the aberrant splicing caused by the IVS1 mutation. There is, however, still room for improvement of the undisturbed expression of the GAA gene in Pompe patients.